linux-kernel - Re: [PATCH] CIFS: Decrease reconnection delay when switching nics

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Date:	Wed, 27 Feb 2013 20:25:04 -0500
From:	simo <idra@...ba.org>
To:	Dave Chiluk <dave.chiluk@...onical.com>
Cc:	Steve French <smfrench@...il.com>,
	Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.org, linux-kernel@...r.kernel.org,
	"Stefan (metze) Metzmacher" <metze@...ba.org>,
	Dave Chiluk <chiluk@...onical.com>
Subject: Re: [PATCH] CIFS: Decrease reconnection delay when switching nics

On Wed, 2013-02-27 at 16:44 -0600, Dave Chiluk wrote:
> On 02/27/2013 04:40 PM, Steve French wrote:
> > On Wed, Feb 27, 2013 at 4:24 PM, Dave Chiluk <dave.chiluk@...onical.com> wrote:
> >> On 02/27/2013 10:34 AM, Jeff Layton wrote:
> >>> On Wed, 27 Feb 2013 12:06:14 +0100
> >>> "Stefan (metze) Metzmacher" <metze@...ba.org> wrote:
> >>>
> >>>> Hi Dave,
> >>>>
> >>>>> When messages are currently in queue awaiting a response, decrease amount of
> >>>>> time before attempting cifs_reconnect to SMB_MAX_RTT = 10 seconds. The current
> >>>>> wait time before attempting to reconnect is currently 2*SMB_ECHO_INTERVAL(120
> >>>>> seconds) since the last response was recieved.  This does not take into account
> >>>>> the fact that messages waiting for a response should be serviced within a
> >>>>> reasonable round trip time.
> >>>>
> >>>> Wouldn't that mean that the client will disconnect a good connection,
> >>>> if the server doesn't response within 10 seconds?
> >>>> Reads and Writes can take longer than 10 seconds...
> >>>>
> >>>
> >>> Where does this magic value of 10s come from? Note that a slow server
> >>> can take *minutes* to respond to writes that are long past the EOF.
> >> It comes from the desire to decrease the reconnection delay to something
> >> better than a random number between 60 and 120 seconds.  I am not
> >> committed to this number, and it is open for discussion.  Additionally
> >> if you look closely at the logic it's not 10 seconds per request, but
> >> actually when requests have been in flight for more than 10 seconds make
> >> sure we've heard from the server in the last 10 seconds.
> >>
> >> Can you explain more fully your use case of writes that are long past
> >> the EOF?  Perhaps with a test-case or script that I can test?  As far as
> >> I know writes long past EOF will just result in a sparse file, and
> >> return in a reasonable round trip time *(that's at least what I'm seeing
> >> with my testing).  dd if=/dev/zero of=/mnt/cifs/a bs=1M count=100
> >> seek=100000, starts receiving responses from the server in about .05
> >> seconds with subsequent responses following at roughly .002-.01 second
> >> intervals.  This is well within my 10 second value.
> > 
> > Note that not all Linux file systems support sparse files and
> > certainly there are cifs servers running on operating systems other
> > than Linux which have popular file systems which don't support sparse
> > files (e.g. FAT32 but there are many others) - in any case, writes
> > after end of file can take a LONG time if sparse files are not
> > supported and I don't know a good way for the client to know that
> > attribute of the server file system ahead of time (although we could
> > attempt to set the sparse flag, servers can and do lie)
> > 
> 
> It doesn't matter how long it takes for the entire operation to
> complete, just so long as the server acks something in less than 10
> seconds.  Now the question becomes, is there an OS out there that
> doesn't ack the request or doesn't ack the progress regularly.

IIRC older samba servers were fully synchronous and wouldn't reply to
anything while processing an operation. I am sure you can still find old
code bases in older (and slow) appliances out there.

Simo.

-- 
Simo Sorce
Samba Team GPL Compliance Officer <simo@...ba.org>
Principal Software Engineer at Red Hat, Inc. <simo@...hat.com>

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